The properties of polymer nanocomposites connected with the size of nanofiller particles are quite different from the properties of composites obtained with fillers with particles of the size above 200 nm. The use of just small amount of the nanofiller in polymer nanocomposites of an order of 0.5-6% permits to improve the mechanical, optical and barrier properties as well as higher chemical and thermal resistance. The coefficient of linear expansion and flammability decrease, which is advantageous for the final product. These results cannot be obtained with the use of standard amounts of the filler (ca. some dozen percent based on the whole composite).